Position of the labrum in agnostid trilobites

Specimens of Peronopsis and Ptychagnostus from the House Range of western Utah provide new information on the location of the labrum (hypostoma) of agnostid trilobites. A labral position as that described here, beneath the second glabellar lobe, has two implications: (1) anomalous cephalic muscle scars described by previous workers may be interpreted as attachment areas for labral muscles; and (2) the mouth probably migrated posteriorly in agnostids. Posterior displacement of the mouth was probably one of several feeding modifications associated with a pelagic mode of life.